Ignore them, report them and then the mods must do their job and ban them asap.
Ignore, ban, repeat as long as necessary until the troll starves and moves on to greener pastures.
Conversely, the Admin can install software hacks that can do some very annoying things to the accounts of trolls, which will also compel them to leave of their own volition...
As a last resort, research can often be done into a user's account which reveals their ISP, who can then be contacted directly by the site's owner or designated agent. Depending upon the nation of origin and the nature of the contract between ISP and enduser, the ISP is often obligated by law to lock out reported 'abusers' from sites reporting such abuse, or they can face legal liability. Of course, once locked out, the offending enduser can simply go to a nearby wifi hotspot, library, etc... However, in my experience, most trolls take the hint that legal action may soon ensue, now that it is clear the owner of the offended site now knows who they are, and that theirs actions are no longer 'anonymous'.
